With reference to FIGS. 1 and 2, from European patent publication no. 0,805,275, a compound vacuum pump is shown having a regenerative pumping mechanism 1 and a molecular drag (Holweck) mechanism 2. Mounted within the pump housing 3 between bearings 4, 5 is a shaft 6. The shaft 6 is adapted for rotation about its longitudinal axis and is driven by an electrical motor 7 surrounding the shaft 6.
The regenerative stage 1 comprises a rotor 9 mounted on the shaft 6. The rotor 9 is in the form of a circular disc, the lower surface of which presents a substantially flat surface on which are positioned integrally therewith a plurality (six) of raised rings 10, 11, 12, 13, 14, 15 situated symmetrically on the rotor about its centre point. Mounted on each of the raised rings is a series of equally spaced blades B, for example, one hundred blades on each ring to form concentric annular arrays of blades. The width of each ring, and the corresponding size of the blades on each ring, gradually decreases from the outermost ring 15 to the innermost ring 10. Each of the blades is slightly arcuate with the concave side pointing in the direction of travel of the rotor.
The body portion 16 of the housing 3 forms the stator and contains six circular channels in its upper surface which are of “keyhole” cross section and are of a size which closely accommodates in the rectangular section upper parts the six raised rings 10, 11, 12, 13, 14, 15; the circular section lower (as shown) parts accommodate the corresponding blades of the relevant raised ring. Each channel has a reduced cross sectional area (not shown) for a small part of its length of a shaped size substantially the same as that of the corresponding blades accommodated therein. This reduced cross sectional part of each channel forms the “stripper” which, in use, urges gas passing through that channel to be deflected by porting (not shown) in to the next (inner) channel until being exhausted from the pump via the bores 32, 33 in the body portion 16.
This arrangement allows for radial sealing between the rotor and stator of the regenerative mechanism. In this respect, the radial sealing occurs between the sides of the raised rings 10, 11, 12, 13, 14, 15 and the corresponding sides of the rectangular cross sectional part of the relevant channel, ie at 17, 18, especially the outermost sides 18, as shown in respect of the ring 10 only to aid clarity in the drawings, due to thermal expansion of the rotor 9 during use of the pump. However, in view of the close tolerances required to effect such radial sealing, any dust or other debris which might accumulate on the outermost sides of the channels 18 through the action of centrifugal forces could, if allowed to build up, cause the pump to seize.
The present invention seeks to solve this and other problems.